1. Field of the Invention
The present invention relates to a structure in a reflective liquid crystal display apparatus or a transflective liquid crystal display apparatus which has reflective functionality.
2. Description of the Related Art
Liquid crystal display apparatuses (hereinafter referred to as “LCDs”) are advantageous in that they are thin and consume relatively little power, and are now widely used for computer monitors and monitors for portable information devices or the like. In LCDs, liquid crystal is sealed between a pair of substrates each having an electrode formed thereon, and the alignment of the liquid crystal disposed between these electrodes is controlled by these electrodes to thereby create a display. However, unlike CRT (Cathode Ray Tube) displays, electroluminescence (hereinafter referred to as “EL”) displays, or the like, LCDs require a light source in order to display an image for viewer observation, because LCDs are not, in principle, self-emissive.
Transmissive LCDs, in which a transparent electrode is used as an electrode formed on each substrate and a light source is disposed on the rear or side of the LC panel, can provide a bright display even in a dark environment, by controlling the transmission amount of light from the light source through the LC panel. Transmissive LCDs, however, have disadvantages in that power consumption is relatively high due to the light source which must continually illuminate, and that sufficient contrast cannot be ensured when the display is used in a bright environment, such as outdoors under daylight.
In reflective LCDs, on the other hand, external light such as sunlight and room light is used as a light source, and such an ambient light entering the LCD panel is reflected by a reflective electrode formed on the substrate provided on the non-viewing surface side. Thus, light enters through the liquid crystal layer, is reflected by the reflective electrode, and then exits from the LCD panel. By controlling the amount of light radiating from the LCD panel for each pixel, reflective LCDs display an image. While reflective LCDs, which use external light as a light source, differ from transmissive LCDs in that their display cannot be seen when no such external light is available, they have advantages that power consumption is very low because the power required for the light source can be eliminated and that sufficient contrast can be obtained in the bright environment such as outdoors. However, reflective LCDs suffer problems in that typical display quality such as reproducibility of color and display brightness is inferior to that of the transmissive LCDs.
On the other hand, because the reflective LCDs having lower power consumption than the transmissive LCDs are more advantageous in response to recent increased demand for lower power consumption devices, attempts have been made to use the reflective LCDs in high resolution monitors or the like of portable devices, and increasing effort has been put in to research and development related to improving the display quality of reflective LCDs.
FIG. 1 is a plan view showing one pixel portion (on a first substrate side) of a conventional active matrix reflective LCD in which a thin film transistor (TFT) is provided for each pixel. FIG. 2 schematically shows a cross sectional configuration of the reflective LCD taken along line C—C of FIG. 1.
The reflective LCD comprises a first substrate 100 and a second substrate 200 which are adhered to each other with a predetermined gap therebetween, and a liquid crystal layer 300 sealed between the first and second substrates. A glass or plastic substrate is used for the first and second substrates 100 and 200, while a transparent substrate is used as the second substrate 200 located on the viewer side, at least in this example.
On a side of the first substrate facing the liquid crystal, a thin film transistor (TFT) 110 is formed for each pixel. In this TFT 110, for example, a drain region in an active layer 120 is connected with a data line 136 which supplies a data signal to each pixel via a contact hole formed in an interlayer insulating film 134. A source region of the TFT 110 is connected with a first electrode (pixel electrode) 150 which is individually formed for each pixel via a contact hole formed to penetrate the interlayer insulating film 134 and a planarization insulating film 138.
A reflective material, such as Al, Ag, or the like, is employed as the first electrode 150. On the reflective electrode 150, an alignment film 160 is formed for controlling the initial alignment of the liquid crystal layer 300.
When the LCD is a color LCD, on a side facing the liquid crystal layer of the second substrate 200, which is disposed so as to oppose to the first substrate 100, a color filter (R, G, B) 210 is formed corresponding to each pixel electrode 150, and a transparent electrode 250 comprising a transparent conductive material such as ITO (Indium Tin Oxide) is formed on the color filter 210 as a second electrode. Further, on the transparent electrode 250, an alignment film 260 which is similar to the alignment film 160 on the first substrate side is formed.
In the reflective LCD configured as described above, the amount of light which enters the liquid crystal panel, is reflected by the reflective electrode 150, and radiates from the liquid crystal panel, is controlled for each pixel, to thereby produce a desired display.
In LCDs, not limited to the reflective LCDs, the liquid crystal is driven by an alternative voltage so as to prevent image persistence. With regard to transmissive LCDs, because both the first electrode on the first substrate and the second electrode on the second substrate should be transparent, ITO is used as a material for both electrodes. Consequently, for AC driving of the liquid crystal, each of the first and second electrodes can apply a positive or negative voltage under substantially the same conditions.
However, in the reflective LCD as shown in FIG. 2, in which a reflective electrode formed by a metal material is used as the first electrode 150 and a transparent electrode formed by a transparent metal oxide material such as ITO is used as the second electrode 250, certain problems such as display flicker and image persistence in the liquid crystal layer may occur depending on the drive conditions. These problems are noticeable when the liquid crystal is driven at a frequency less than the critical flicker frequency (CFF), for example, as has been reported recently. In order to further reduce power consumption of LCDs, attempts have been made to reduce the frequency for driving the liquid crystal (≈ the frequency for writing data to liquid crystal (liquid crystal capacitor) at each pixel formed in the region where the first and second electrodes face each other) equal to or less than the CFF at which image flicker can be recognized by the human eye, approximately 40 Hz–30 Hz, by reducing such a drive frequency to less than 60 Hz, which is a reference frequency in the NTSC standards, for example. It has been revealed, however, that when each pixel of a conventional reflective liquid crystal panel is driven at a frequency less than the CFF, the above-described problems of flicker and image persistence in the liquid crystal layer are significant, which leads to significant deterioration in display quality.
The applicant's research into the causes of such flicker and image persistence in the liquid crystal layer generated in a reflective LCD as shown in FIGS. 1 and 2 revealed that asymmetry of the electrical characteristics of the first and second electrodes relative to the liquid crystal layer 300 is one cause. It is believed that such asymmetry results from a significant difference between a work function of 4.7 eV–5.2 eV for the transparent metal oxide such as ITO used in the second electrode 250 and a work function of 4.2 eV–4.3 eV for the metal such as Al used in the first electrode 150. Any such difference in the work function would cause there to be a difference of charge actually induced on the liquid crystal interface via the alignment films 160 and 260, when a same voltage is applied to each electrode. Such a difference of charge induced on the interface between the liquid crystal and the alignment film at each electrode side would in turn cause impurity ions or the like to be unevenly located toward one of the first and second electrodes within the liquid crystal layer, which results in accumulation of remaining DC voltage in the liquid crystal layer 300. As the liquid crystal drive frequency is lowered, the influence of this remaining DC voltage on the liquid crystal increases and generation of flicker and image persistence in the liquid crystal layer become more significant. Accordingly, driving the liquid crystal at a frequency less than the CFF, in particular, is substantially difficult.
Reflective LCDs in which ITO is used for both the first and second electrodes as in transmissive LCDs and a reflector is separately provided on the outer side of the first electrode (on the side of the first electrode not facing the liquid crystal) are conventionally known. When a reflector is thus provided on the outer side of the first substrate, however, the length of a light path is increased by an amount corresponding to the thickness of the transparent first electrode 150 and of the transparent first substrate, thereby making the display quality likely to deteriorate due to parallax. Consequently, in reflective LCDs which demand high display quality, a reflective electrode is employed as a pixel electrode, and it is therefore impossible to reduce the drive frequency so as to achieve lower power consumption, because flicker or the like is generated at the lower drive frequency, as described above.